The present invention relates to a castor wheel construction, particularly designed for furniture pieces and the like.
As is known, swinging or castor wheels are conventionally used for allowing furniture pieces such as chairs, tables, desks and carriages, as well as shop trolleys, to be easily handled.
Prior castor wheels may be classified as included substantially into two classes, that is a single wheel type or a double wheel element type.
In a single wheel element castor wheel, a single wheel element has a rotary axis cooperating with a support element which, in turn, is clamped or connected to a furniture piece or a trolley, by a vertically extending connecting pin.
This connecting pin allows the castor wheel to rotate about a substantially vertical axis, thereby allowing the furniture piece or trolley to be driven in any desired direction.
The individual or single wheel element castor wheels have the disadvantage that they provide a resistance against a rotary movement of the wheel assembly about the vertical axis, thereby the movement of the furniture piece or trolley the wheel assembly is applied to cannot be considered an even movement.
Actually, for allowing the wheel assembly to be rotated about the vertical axis, the wheel element must be able of rotating about a line passing through the wheel element and its floor contacting point.
In fact, a peripheral portion of the wheel element tends to slip on the floor, instead of freely rotating.
Moreover, the wheel element central rotary axis is clamped outside of the wheel element by clamping or support members which are not protected with respect to possible impacts against articles such as other furniture pieces or walls.
Such an impact could damage the wheel assembly axis, thereby decreasing the operating efficiency of the wheel assembly, or making a proper operation impossible.
The double wheel element systems, on the other hand, provide to use two wheel elements having a shared rotary axis which is fixed to the wheel assembly support element at an intermediate position between the two wheel elements, which are free of separately rotating.
Thus, the wheel elements may also rotate in mutually opposite directions, thereby facilitating the overall rotary movement of the wheel assembly about the vertical rotary pin fixing the wheel assembly to the furniture piece or trolley, and greatly improving the evenness of movement and the direction changing capability.
Moreover, in the double wheel element system, the wheel element axis support is arranged at the center of the support assembly and between the two wheel elements for protecting them from possible impacts and damages, thereby causing a consequent driving efficiency decrease.
A main problem of the above two wheel element assembly is that of properly clamping the wheel elements to the central axis.
Differently from the first class wheel assemblies, in which the central axis is coupled to the two opposite faces of the wheel element, in the double wheel element system, each wheel element is respectively connected to an opposite end portion of the central axis, with the axis support arranged between the two wheel elements.
This connecting or clamping system is not efficient as desired, since it has a very low stability: in fact, since the axis does not pass through the wheel element, the wheel element load on said axis is an asymmetrical one, and being maximum near the wheel element inner part joined to the axis and minimum, or zero, outside of the wheel element.
Such instability generates, as it should be easily apparent from an examination of a wheel assembly of the second type, a great clearance between the wheel elements and central axis, which not only causes the wheel assembly to inefficiently operate, but which, moreover, negatively affects the aesthetic aspect of the overall furniture piece or trolley the wheel assembly is connected to.
Both the above disclosed castor wheel types, moreover, have further substantial drawbacks.
In fact, in both the above castor wheel types, the wheel element proper rotates about its rotary axis and the rotary movement is allowed owing to a very low friction coefficient between the wheel element and axis.
Since the diameter of the wheel element, with respect to the rotary axis engagement hole is a typically large one, then the torques operating on the axis-wheel coupling hole are correspondingly large.
Due to the above reasons, upon a long use of the wheel assembly, friction will progressively remove material from the hole thereby enlarging the latter, to provide an instable rotary movement on the wheel on its axis, with a consequent inefficient operation of the overall wheel assembly.
Moreover, in a condition in which the rotary of a wheel element is a comparatively slow one and the main function of the wheel element is that of supporting a comparatively large load, then the use of a central rotary axis does not constitute the most efficient approach.
In fact, during the rotary movement, the wheel elements transmit a torque about the line passing through the center of the wheel element and perpendicularly to the movement direction.
Such a torque is generated since the force applied on the wheel element at its floor contact point is multiplied time its distance for its rotary axis, that is the wheel element radius.
Such a torque provides an additional load on the wheel element/axis assembly, thereby further decreasing the operating stability of the overall wheel assembly.
So-called hubless castor wheels are also known.
In these hubless castor wheels, a supporting element comprises an inner annular portion thereabout a toroidal wheel element is caused to turn through an interposition of sliding elements.
These sliding elements, which may comprise sliding balls or rolls, are engaged in sliding recesses formed both on the annular inner portion and on the toroidal wheel inner part.
Thus, since they are frontally facing, they will hold inside the wheel assembly the sliding elements to allow the latter to properly operate.
From the above it occurs that, for making a hubless castor wheel, it is necessary to form the above mentioned recesses, with a comparatively high machining precision, causing in turn an increase of the making and selling cost of such a hubless castor wheel.
Moreover, in a long operation time, the sliding elements tend to damage the recessed contours to enlarge them, and correspondingly enlarging the clearance between the annular inner element and toroidal wheel, thereby decreasing the stability of the overall wheel assembly.